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Landfill gas is produced by the breakdown of household garbage by bacteria and typically consists of 40-75% carbon dioxide and 25-65% methane by volume, plus trace amounts of other gases, including volatile organic compounds (VOCs) (Peteranecz & Kear, 1992; US EPA, 1991). Landfill gases, including methane, can easily move though permeable soils like those present at the NSL site. These gases usually move from areas of high pressure (at depth within the landfill) to areas of low pressure (surficial soils and atmosphere), but it is often difficult to determine specific patterns of gas movement. The gas seeks the path of least-resistance, often preferentially following sand or gravel-filled utility trenches once off-site .

High concentrations of methane occur most commonly in landfills that contain municipal garbage and have an engineered landfill cap, trapping these gases beneath an impermeable cover. The landfill cap prevents the gases from escaping upward, causing them to move laterally into adjacent, off-site land areas. Highest methane concentrations occur in the warmer summer months and concentrations are higher during the heat of the day compared to measurements taken during morning hours (Van Schiver, 1992). Methane levels in soils tend to be higher during dry periods and lower after significant rainfall events. Associated with high methane production are increases in carbon dioxide and hydrogen sulfide and decreased amounts of oxygen (Van Schiver, 1992).

Methane is a flammable, potentially explosive gas that is combustible only under specific conditions, requiring the right combination of methane and oxygen plus a source of ignition. Methane is explosive at concentrations that range from 5 to 15% methane per volume of air. This corresponds to methane concentrations of 10,000 to 30,000 parts per million (ppm). There has been at least 30 reported cases of explosions associated with landfill gases, causing property damage and killing or injuring nearby residents or workers (US EPA, 1991).

The lower explosive limit (LEL) of methane corresponds to methane levels of 5% methane per volume of air. At concentrations below the LEL, the methane/air mixture is too dilute (methane concentrations are too low) to ignite. Any concentration between the LEL and the upper explosive limit (UEL) of methane (=15% methane/v) has the right combination of methane and air to cause combustion of the gas (US EPA, 1991) if a source of ignition is available. Methane concentrations above the UEL (>15%/v) are too rich (oxygen levels are too low) to support combustion. To sustain a flame, oxygen levels have to be at or above 19%.

The Explosive Gas Monitoring system installed at the NSL site appears to be adequate based on current conditions. All current and historical data regarding gas levels at the NSL site reflect cool, wet climatic conditions that are not optimal for the production of methane. Daily monitoring of gas probes will become more critical as temperatures rise and rainfall levels decrease in August and September when the hot, dry conditions optimal for methane production occur.

The Landfill Gas Abatement Plan proposed by the VLSG and its contractor appears to be adequate as a short-term fix. The efficency of the planned system with regard to collecting landfill gases and preventing their migration off-site remains to be seen. A possible problem with the passive vent system proposed by the PRP group is the low gas head inferred for the landfill by the VLSG contractor. Gas pressures may not be sufficient to fully vent the landfill gases through a passive system.

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